The present disclosure relates to an auto focusing method using a VCM Voice Coil Motor) actuator in a camera or a camera module.
It is recently essential for a camera or a camera module to be mounted with an auto focusing function.
The auto focusing control function is to detect a harmonic component of an image obtained from an image sensor to obtain a focus value (FV) useful to the focus adjustment, and to move a lens to a point where the FV is a maximum value. The method may include a part that detects the FV, a part that determines a moving position of the lens by evaluating the FV, and a part that controls the lens.
Normally, a hill-climbing search algorithm is used to find this maximum focus value. Information on focus control is obtained from an image signal obtained without any separate devices.
A voice coil motor (VCM) is an actuator that uses Lorentz force generated by electromagnetic force of a coil in a magnetic field of a permanent magnet to linearly move a lens. The VCM is adequate to build a system configured to linearly and accurately move a lens in a relatively short distance. That is the VCM has advantages in terms of a short response time suitable for translation motion, a relatively long stroke suitable for miniaturization, and high-precision position control. However, several parameters such as the lens weight, lens size and lens inertia cause a time delay in the lens movement to a desired target position. In particular, hysteresis characteristics of the voice-coil motor and lens inertia cause a generation of different focus values under identical conditions.
That is, in a case an auto focusing (AF) algorithm is implemented using the VCM actuator, hysteresis characteristics of the voice-coil motor are generated in which different focus values under identical conditions are caused, and an upward position value and a downward position value of VCM are differentiated when auto focusing command is received due to structural characteristics of VCM.
To be more specific, the VCM also suffers from disadvantages in that it can hardly expect its optimal control performance with the generally used controller, because it has a nonlinear characteristic, a time-varying characteristic in which its dynamic characteristic varies with the passage of time, and a hysteresis characteristic
In the AF algorithm, an AF position code value that is accepted as the most optimal is taken by the upward characteristic of the VCM to set up a lens position based on the AF position code value in the downward characteristic. However, the disadvantage is that an accurate AF cannot be implemented due to a difference between the upward position value and the downward position value.